Steady state electrolyte fluxes across in vitro rabbit ileum.

Parallel pathways for ion movement exist in rabbit ileum. A high-conductance cation-selective extracellular pathway parallels the low-conductance transcellular route. The extracellular pathway comprises a series array of lateral cell space and tight-junction. Aspects of ion flux through these two pathways have been studied. The cation content of strips of rabbit ileum has been measured. The [Na] in the tissue extracellular (inulin) space was calculated to be 40mM hypertonic to the bathing medium (140mM Na). This result provides evidence that the lateral intercellular spaces of rabbit ileum are 80 m.osmoles hypertonic to the bathing medium. 2,4,6, triaminopyrimidine reduces the Na conductance of the extracellular pathway and has been used as a means of separating transcellular from extracellular Na movements. A portion of net Na flux in control tissues is due to asymmetry generated in the extracellular pathway; this may result from an osmotic pressure gradient across the tight-junction due to lateral-space hypertonicity. Measurement of the tissue isotope specific activity ratio together with the measurement of bidirectional transcellular Na or C1 fluxes allows calculation of the four unidirectional ion fluxes across the mucosal and serosal boundaries of the transcellular pathway. The effects of D-galactose, a sugar actively transported by ileum, upon transcellular unidirectional ion fluxes has been examined. The coupling between galactose and Na fluxes is discussed in relation to models for active sugar transport. The effect of the secretagogue, theophylline, upon Na and C1 fluxes was tested. Triaminopyrimidine reverses the theophylline-stimulated secretion of Na, indicating that Na secretion occurs via the tight-junctions. Theophylline causes an increased passive permeability of the mucosal border to C1. Net secretion of NaC1 in the presence of theophylline may result from passive movement of NaC1 from the hypertonic lateral space to the mucosal solution.